The present invention relates generally to electronically controlled fuel injection systems of the type in which the quantity of fuel to be fed to an internal combustion engine is computed and injected accordingly, and more particularly the present invention relates to an improvement in the temperature fuel enrichment characteristic of such fuel injection system.
In known fuel injection systems of the above type, the duration of the opening of injection valves (hereinafter simply referred to as an injection time) is determined in accordance with the temperaturedependent variations of the resistance value of a temperature sensor provided in the cooling water path of an internal combustion engine (hereinafter referred to as an engine) to thereby control the fuel injection quantity to suit the required temperature fuel enrichment characteristic of the engine. However, there is a disadvantage in that since the temperature fuel enrichment characteristic required for the engine when it is left under the idling condition differs from that required under the load condition, if the temperature fuel enrichment characteristic is adjusted to ensure smooth idling of the engine, the fuel injection system operated in accordance with this temperature fuel enrichment characteristic tends to feed an excessive quantity of fuel during the driving period prior to the completion of warming up the engine, thus increasing the fuel consumption rate.
Another disadvantage is that if the temperature fuel enrichment characteristic is established to suit the fuel requirements of the engine under the load conditions, the engine tends to stall when it is returned to the idling speed.
While most common types of fuel injection systems which have heretofore been proposed to overcome the foregoing difficulty include those in which the temperature fuel enrichment characteristics are changed when the pressure in the engine air intake passage exceeds a predetermined value (i.e., when the load increases) or when the vehicle speed reaches a predetermined value as well as others in which the characteristics are changed in accordance with the openings of the throttle valve, there are disadvantages that the driveability is impaired by a rapid change in the fuel injection quantity resulting from the stepwise change of the characteristics and that the provision of an additional pressure switch, vehicle speed discriminator, throttle switch or the like is required.